Vacuum circuit breakers can shut down high current while their sizes are kept small, so they are used in electric power transformation facilities and power distribution facilities. It has been also considered that two or more vacuum circuit breaker units are connected in parallel to have a large capacity so that higher current can be shut down with the vacuum circuit breakers.
In a structure in which two or more vacuum circuit breaker units are simply connected in parallel, however, when a single common operating unit is used to operate these vacuum circuit breaker units to shut down current, a shutdown delay is caused among the vacuum circuit breaker units due to a slight difference between their mechanisms or another reason. When this happens, the vacuum circuit breaker unit that caused a delay in opening the electrodes has to shut down the total current, significantly damaging the electrodes and leading to the inability to use the vacuum circuit breaker unit.
A vacuum circuit breaker for addressing the above problem is proposed in JP 04-274119 A (1992) (Patent Document 1). The vacuum circuit breaker in Patent Document 1 is a large-capacity vacuum circuit breaker that is structured so that two vacuum circuit breaker units are connected in parallel; one of these units uses vertical magnetic field electrodes and the other uses flat-plate electrodes, a current limiting device made of, for example, ceramic being connected in series to the vacuum circuit breaker unit that uses the flat-plate electrodes, the current limiting device having a positive temperature coefficient of resistance. Alternatively, the proposed large-capacity vacuum circuit breaker is structured so that a vacuum circuit breaker unit that uses vertical magnetic field electrodes and a spiral-type vacuum circuit breaker unit that generates high vacuum arc voltage are connected in parallel.